Our goal is to characterize the mutation spectra and identify potential molecular markers of low-linear energy transfer (LET) radiations (gamma-rays) and high-LET radiations (neutrons). Ionizing radiations are known to generate many kinds of DNA lesions as a result of damages to sugars, bases and sugar-phosphate backbone of DNA molecules. The lesions and processes that underlie mutation and cancer, however, remain unknown. We hypothesize that ionizing radiations will induce a wide spectrum of mutations including point mutations, deletions of various sizes and complex rearrangements and that the spectrum will be a function of radiation dose, dose-rate and quality. This hypothesis can be tested comprehensively using our newly developed polymerase chain reaction (PCR)-based procedure coupled with conventional Southern blot hybridization and molecular cytogenetic procedures to characterize mutants induced by 60-Co gamma-rays and by fission-spectrum neutrons. These procedures are capable of detecting a full range of molecular events associated with mutation. Using the unique radiation facilities at Argonne National Laboratory, capable of irradiating cells at very low dose rates, mutations will be analyzed in the Chinese hamster ovary cells(clone Kl-BH4)/hypoxanthine-guanine phosphoribosyltransferase (CHO/PRT) assay. The CHO/HPRT assay was chosen because it has been well characterized at the cellular and molecular level and we have developed a large cellular mutagenesis database for low- and high-LET radiations. Further, our PCR procedure is capable of detecting a wide spectrum of mutations in a large number of mutants. Our proposed research has three specific aims: (1) to study the effects of the dose and dose rate on gamma-ray-induced mutation spectra; (2) to determine the influence of dose and dose rate on neutron-induced mutation spectra and compare it to the gamma-ray spectra; (3) to investigate the role of DNA repair in radiation mutagenesis by studying the modulation of the mutation spectra induced by gamma-rays and by neutrons in a CHO-K1 cell derivative xrs-5, which is defective in DNA double-strand break rejoining ability.